25-08-2017, 09:32 PM
TWO AND FOUR WHEELER CARBURETOR
FOUR WHEELER .doc (Size: 677.5 KB / Downloads: 22)
What is carburetor?
A carburetor carburetor, or carburetor is a device that blends air and feul for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom.
Word origin:
The word carburetor comes from the French carbure meaning "carbide”. Carburer means to combine with carbon In fuel chemistry, the term has the more specific meaning of increasing the carbon content of a fuel by mixing it with a volatile hydrocarbon.
History:
A carburetor was developed by Enrico Bernardi at University of Padua in 1882, for his “Motrice Pia”, the first petrol combustion engine prototyped on 5 August 1882.
A carburetor was among the early patents by Karl Benz as he developed internal combustion engines and their components. The Austria automobile pioneer Siegfried Marcus invented the “rotating brush carburettor”. This was further improved by the Hungarian engineers János Csonka and Donate Bánki in 1893.
Carburetors were the usual fuel delivery method for most U.S. made gasoline-fueled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last carbureted cars were:
1990 (General public) : Oldsmobile Custom Cruiser, Buick Estate Wagon
1991 (Police) : Ford Crown Victoria Police Interceptor with the 5.8 L engine.
1991 (SUV) : Jeep Grand Wagoneer with the AMC 360 engine.
1994 (Light truck) : Isuzu
Principles:
The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.
Component:
Power valve: For open throttle operation a richer mixture will produce more power, prevent pre-ignition detonation, and keep the engine cooler. This is usually addressed with a spring-loaded "power valve", which is held shut by engine vacuum. As the throttle opens up, the vacuum decreases and the spring opens the valve to let more fuel into the main circuit. On two-stroke engines, the operation of the power valve is the reverse of normal — it is normally "on" and at a set rpm it is turned "off". It is activated at high rpm to extend the engine's rev range, capitalizing on a two-stroke's tendency to rev higher momentarily when the mixture is lean.
Accelerator pump:
The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, most carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts.
Choke:
To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls extra fuel through the main metering system to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.
In many carbureted cars, the choke is controlled by a cable connected to a pull-knob on the dashboard operated by the driver. In some carbureted cars it is automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat, or to an electric heating element. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle's accelerator is moved to the end of its travel. This provision allows a "flooded" engine to be cleared out so that it will start.